Of the cellular components of the hematopoietic microenvironment, the endothelial cell defines the interface between the vascular and hemopoietic compartments, and its role in the regulation of hemopoiesis has until recently, not been well studied. My laboratory has reported the isolation and characterization of bone marrow microvascular endothelial cells (BMEC) and their ability to sustain long term multilineage hemopoiesis. Using a BMEC-derived expression library we have identified a secreted, glycosylated form of L-ferritin with a plasma concentration in the picomolar range, that is active in supporting the proliferation of pluripotent stem cells. Interestingly, like many hematopoietically active cytokines, glycosylated L-ferritin is a member of the four-helix bundle family of proteins. The biology of this newly recognized stem cell active molecule is the subject of this proposal. Our data suggest that glycosylated L-ferritin is secreted by bone marrow microvascular endothelial cells and supports stem cell self renewal. This is suggested by our expression cloning data as well as by preliminary studies with purified recombinant glycosylated L-ferritin. The activity of glycosylated L-ferritin is synergistic with kit-ligand (KL). To explore the biology of this molecule and the mechanism by which it signals, we will define the biology of glycosylated L-ferritin with respect to its support of progenitor survival and expansion in proliferation assays, in a model of marrow reconstitution, and in studies designed to shed light on its role in the developmental biology of hematopoiesis. We will define the mechanism of glycosylated L-ferritin signaling in studies that will define the nature of its interaction with cells. The mechanism of glycosylated L-ferritin's synergy with KL will be explored using transfection models to define signal transduction events that accompany glycosylated L-ferritin binding. The receptor responsible for glycosylated L-ferritin binding and signaling will be defined. These studies will define the biology of a newly recognized hematopoietic cytokine of potential therapeutic importance.